Frequency-shift keying (FSK) is a commonly-used frequency modulation scheme in which information is transmitted over a communications link by way of discrete frequency changes made to a carrier signal. FSK transmitters are inexpensive to manufacture and have inherently high efficiency. However, FSK demodulators, which are needed to demodulate the FSK modulated signals at the receiving end of the communications link, have several disadvantages.
One disadvantage relates to the fact that conventional FSK demodulators are difficult to manufacture in integrated circuit (IC) form. Forming the FSK demodulator in an IC is desirable since it lowers manufacturing costs and results in a compact design that consumes significantly less power than a nonintegrated implementation. Unfortunately, conventional FSK demodulators include circuit components that are difficult to integrate using standard IC fabrication processes. For example, many FSK demodulators include slope detectors, ratio detectors or quadrature multipliers, all of which employ some sort of high-Q tuned analog circuit. Incorporating these high-Q tuned analog circuits in standard IC fabrication processes is difficult, and usually results in substantial yield losses and hard-to-control and undesirable part-to-part performance variations.
Other FSK demodulation approaches employ a monostable integrator or a delay flip-flop (DFF). The monostable integrator approach requires an accurate pulsewidth of a small fraction of the demodulation carrier frequency. The level of accuracy required makes it difficult to integrate. The DFF-based approach, while more easy to integrate than the other approaches, is only capable of operating on signals having a very high FSK modulation index h (i.e., an h much greater than 1). The DFF-based FSK demodulator includes a quadrature demodulator which serves to control the logic output of the DFF depending on whether the frequency of the received FSK modulated signal is lower than or higher than a local oscillator frequency. In order for the quadrature demodulator to accurately generate the control signals for the DFF, there must be sufficient phase rotation during each data bit interval of the received FSK modulated signal. However, in low-modulation index applications, such as Bluetooth where the modulation index is only about 0.3, insufficient phase rotation may be available per data bit interval for the DFF-based FSK demodulator to work properly. Another limitation of the DFF-based FSK demodulator is that the bit rate must be maintained at a rate less than or equal to the FSK frequency deviation imposed on the carrier signal. These constraints limit practical application of the DFF-based FSK demodulator to low-data-rate, high-modulation-index applications.
In addition to the specific problems associated with the various FSK demodulation approaches discussed above, all FSK demodulators exhibit a phenomenon known as the “threshold effect.” At a pre-demodulation SNR (or “input SNR”) called the “FM threshold,” the post-demodulation SNR (or “output SNR”) begins to degrade much more rapidly than the pre-demodulation SNR. Because a low output SNR results in data errors at the output of the demodulator, it is highly desirable to extend the onset of this threshold. Unfortunately, as the input SNR decreases, it becomes increasingly more difficult to extend the threshold, due to the presence of what are known as “clicks”. Clicks are noise events that enhance the additive noise generated in the demodulation process. At low input SNRs the resulting noise enhancements become the dominant noise source and, consequently, pose a limit on the ability to extend the onset of the threshold effect.
Considering the foregoing drawbacks and limitations of prior art FSK demodulation approaches, it would be desirable to have an FSK demodulator that is amenable to integration, capable of operating on both low and high modulation index signals, and effective at extending the onset of the threshold effect.